Gay men are often asked, 'When did you first realise you were gay?' But exactly the same question applies to heterosexuals. Even in 1948, more than half of the men in Alfred Kinsey's famous sample recalled mutual masturbation with other boys in childhood. From this perspective, adult gaiety could be seen as a natural extension of childhood tendencies.

While questioning the causes of gaiety, we should be asking at what point heterosexuals focus their desires exclusively on women. But the causes of gaiety are not just any old scientific question. Findings in favour of nurture or nature are liable to be leapt upon by homophobes and homophiles alike. This was proven by a large survey of attitudes in several developed nations.

Homophobes tended to see homosexuality as a choice and the result of upbringing, whereas homosexuals and their supporters saw it as a genetically inherited preference. Believing in genes removes any possibility of 'blame' falling on parents. It also becomes an unchangeable, natural destiny - not a choice that homophobes could portray as an illness which might be treated with therapies.

What is more, men can get very aroused by the subject, literally so in homophobes. A clever study tested Freud's claim that homophobia can be a defence against homosexual desires, a reversal of true feeling. Heterosexual men were given

a questionnaire measuring their homophobia levels, then shown erotic homosexual films. They also had their penises attached to a device that measures tumescence. Sure enough, those rated as homophobes were more likely to tumesce when watching the film: the more they were conscious of detesting homosexuality, the more likely they were to desire other men.

It is interesting in itself that so much research money has been spent on searching for the causes of gaiety. I suspect that far more has been expended on this relatively rare predilection (at most, it is 3 per cent, not 10 per cent of men who are exclusively gay) than on explaining heterosexual practices. Its decriminalisation and removal from the list of mental illnesses may have made it less easy to obtain funding for studies into the subject, but there are still many done.

As the younger generation shows increasingly unprejudiced attitudes towards gays, hopefully evidence of causation will slip from being high-calibre munitions to be fired by homophobes to one of many scientific questions about the causes of different sexualities.

For, once you remove yourself from the ideological implications, getting away from the idea that nurture of a gay man is a Bad Thing, you can focus on a group of studies whose results are most intriguing. They do, in fact, show that parental care is a major factor in causing gaiety but that should not entail any opprobrium towards anyone at all, any more than liking heterosexual sex standing up, or mid-afternoon, or in aircraft lavatories should.

· Next week: the causes of gaiety. Oliver James is unable to enter into any personal correspondence.